I'm not clear about what happened here - when did you start noticing the loss of compression? Did you adjust the timing chain and then have the problem or did you adjust the timing chain because of the problem?
If you didn't "take care of the decompresion mechanism well" that seems like the obvious first place to check.
Check your valve clearances again to make sure they're okay. Do you have a feeler gauge?
When did you last adjust your valves?
You might have adjusted them improperly, but that doesn't seem likely since you would probably notice the loss of compression right away. Make sure you DO NOT turn the crankshaft backwards when you're setting it to top dead center - if you pass it, keep turning again until you reach TDC.
Wright
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Wright Bagwell
http://www.geocities.com/wwbagwell/